1. Field of the Invention
This invention relates to electrical distribution systems. More particularly, this invention relates to a zero phase sequence harmonic current filter apparatus and method for connection to the load-end of a six-wire or four-wire branch circuit.
2. Description of the Background Art
High levels of positive, negative and third order, zero [phase] sequence harmonic currents are generated by the single-phase, non-linear loads that are connected between phase and neutral in a three-phase, four-wire distribution system. Although each single-phase, non-linear load is normally small, they often represent 95% to 100% of all loads connected to a low voltage distribution system in an office, data center or audio-video facility.
Unlike the positive and negative sequence phase currents, which are each displaced by 120.degree. in a three-phase system, zero sequence phase currents are each displaced by 0.degree. and are therefore "in-phase". As a result, zero sequence phase currents combine arithmetically at the source transformer's neutral terminal (X.sub.0) and return to the nonlinear loads via the system's neutral conductor. In a worst case scenario, the resulting zero sequence neutral currents may be greater than 1.5 times the phase currents.
Zero sequence phase currents, acting in an Ohm's Law relationship with the system's zero sequence impedances, produce zero sequence voltages. These zero sequence voltages distort the fundamental voltage waveforms.
The most significant source of third-order, zero sequence currents is the switch-mode power supply. These power supplies are found in personal computers, mainframe terminals, monitors, LAN controllers-servers, printers, photocopiers, facsimile machines, electronic ballasts, television sets, audio-video amplifiers and recorders.
Significant levels of third-order, zero sequence currents and voltages in the electrical distribution system will have a severe impact on both the system and the devices connected to it. Depending on the capacity and configuration of the system, the presence of third-order, zero sequence currents may include any or all of the following symptoms:
a) High peak phase currents, PA1 b) High average phase current, PA1 c) High total harmonic distortion of current, PA1 d) High total harmonic distortion of voltage, PA1 e) High system losses, PA1 f) Apparatus overheating, PA1 g) High neutral current, PA1 h ) High common mode noise, PA1 i) Low power factor, and PA1 j) High cost of power.
In addition to the higher operating and maintenance costs associated with poor power quality and common mode noise, overloaded system neutral conductors, branch circuit "shared neutral" conductors and office partition "shared neutral" conductors may pose a serious fire and safety hazard.
Various techniques have been used to mitigate these symptoms. These include the replacement of conventional and K-Factor rated distribution transformers with specialized transformers that have ultra-low zero sequence impedance, the application of zero sequence shunt filters at distribution panels and/or sub-panels, and the application of directional zero sequence series filters at the line side of distribution panels.
More particularly, with reference to FIG. 1, a conventional four-terminal zig-zag autotransformer, which is applied to a three-phase, four-wire electrical distribution panel or sub-panel as a zero sequence current filter, has six windings: 1, 2, 3, 4, 5 and 6 respectively. Normally, each of these windings has an equal number of turns. The six windings are installed on a three-phase magnetic core which has three core legs: a, b and c respectively.
Windings 1 and 2 are installed on core leg a, windings 3 and 4 are installed on core leg b, and windings 5 and 6 are installed on core leg c.
The three phases of the electrical power distribution system are connected to filter terminals 10, 30 and 50, and the neutral conductor of the electrical power distribution system is connected to filter terminal 70.
Filter terminal 10 is connected to winding 1 at junction 11. Connecting junctions 12 and 62 connects winding 1 to winding 6. Winding 6 is connected to terminal 70 at junction 61.
Filter terminal 30 is connected to winding 3 at junction 31. Connecting junctions 32 and 22 connects winding 3 to winding 2. Winding 2 is connected to terminal 70 at junction 21.
Filter terminal 50 is connected to winding 5 at junction 51. Connecting junctions 52 and 42 connects winding 5 to winding 4. Winding 4 is connected to terminal 70 at junction 41.
Connected in this fashion and under balanced zero sequence current conditions, the zero sequence currents, which flow through each pair of windings on the common core leg, will be equal but of opposite polarity. The flux produced by each of these windings will also be equal and have opposite polarity. As a result of flux cancellation, the zero sequence impedance of the filter will be reduced to the resistance of the filter's winding conductors.
The zero sequence impedance of an ideal filter will normally be at least ten times lower than that of the power source. By connecting the filter in parallel with the power source and the single-phase, non-linear loads, the load-generated zero sequence currents will be attracted by the lower impedance of the filter. This will result in a reduction of the zero sequence currents in the three-phase, four-wire system between the filter connection and the power source.
Known prior art zero phase sequence harmonic current filters are disclosed in U.S. Pat. Nos. 5,406,437, 5,416,688, and 5,576,942 and in Power Quality, Sept./Oct. 1991, pp. 33-37, "Eliminating Harmonic Currents Using Transformers." by Robert H. Lee of R. H. Lee Engineering. Further, zero phase sequence harmonic current filters have been discussed in technical papers presented at the IEEE IAS Annual Meeting, Orlando, Fla., held October 1995, by Thomas Key and Jih-Sheng Lai entitled "Costs and Benefits of Harmonic Current Reduction for Switch-Mode Power Supplies in a Commercial Office Building." and at the NETA Annual Conference, Mar. 19, 1997, Gregory N. C. Ferguson, Power Quality International, Inc. entitled "Power Quality Improvement in a Harmonic Environment." The disclosures of the above-referenced patents, publications and technical papers are hereby incorporated by reference herein.
As set forth in the forgoing disclosures, although these prior art devices can be effective in mitigating these symptoms, at their point of application or between their point of application and the upstream source transformer, they cannot relieve these symptoms in the downstream branch circuits or at the branch circuit loads, particularly where the branch circuits are long. Indeed, with regard to all patented and/or commercially available zero [phase] sequence current filters known to Applicant, they are configured as three-phase, four-wire devices and are therefore entirely unsuitable for application at the load-end of a branch circuit which includes three pairs of phase and neutral conductors (six-wire). Therefore, there has been a long-felt need for adapting prior art zero phase sequence harmonic current filters to the load-end of a branch circuit which includes three pairs of phase and neutral conductors (six-wire).
Therefore, it is an object of this invention to provide an improvement which overcomes the aforementioned inadequacies of the prior art devices and provides an improvement which is a significant contribution to the advancement of the zero sequence harmonic filter art.
Another object of this invention is to provide zero phase sequence harmonic current filter apparatus and method for connection to the load-end of a six-wire or four-wire branch circuit.
Another object of this invention is to provide a zero phase sequence harmonic current filter apparatus and method that significantly reduces peak phase currents, average phase currents, total harmonic distortion of current, total harmonic distortion of voltage, system losses, apparatus overheating, neutral current, common mode noise, low power factor, and cost of power at the load-end of a branch circuit which includes three pairs of phase and neutral conductors (six-wire).
Another object of the invention is to provide an apparatus and method for reducing the non-linear load-generated zero sequence harmonic currents and voltages on three single-phase circuits, which are combined to form a six-wire branch circuit, and their three-phase, four-wire distribution system source, said apparatus and method comprising a three-phase zig-zag autotransformer, having three pairs of phase and neutral terminals and means for respectively connecting the phase and neutral terminals of the zig-zag autotransformer in parallel with the six-wire branch circuit at the load-end thereof.
Another object of this invention is to provide an apparatus and method for reducing the non-linear load-generated zero sequence harmonic currents and voltages on three single-phase circuits, which are combined to form a four-wire, "shared neutral" branch circuit, and their three-phase, four-wire distribution system source, said apparatus and method comprising a three-phase zig-zag autotransformer, having three pairs of phase and neutral terminals; means for connecting the three neutral terminals to the shared neutral of the branch circuit; and means for respectively connecting the phase terminals of the zig-zag autotransformer in parallel with the three phases of the branch circuit at the load-end thereof.
The foregoing has outlined some of the pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.